The plasma spraying of metallic, ceramic, and other coatings onto a substrate material has long been used to create critical mechanical parts having a coating of a hard wear or heat resistant material overlaid onto a strong ductile material. The resulting composite provides a structural component that has good mechanical properties such as strength and ductility and also has a surface that is resistant to corrosion and/or heat stress caused by rapid changes in temperature. Rocket engine turbine blades, for example, are traditionally plasma spray coated with an appropriate ceramic that can withstand the rapid temperature changes that occur when the engine is started and shutdown. In other applications, plasma spray techniques have been used to replace material that may have worn away from a component part. Plasma spray techniques have also been used to build up a thick coating of material over a preformed mold, thus actually fabricating a component from the sprayed material itself. Other advantageous applications of plasma sprays have also been made.
The plasma spraying of coatings generally is achieved by means of a plasma spray device such as a gun. While such devices can vary greatly in their operational details, their fundamental elements usually include a passageway through which an inert gas or air is expanded, often to supersonic velocities. A cathode usually is provided at the upstream end of the passageway. A high current arc is electrically induced between the cathode and the walls of the passageway, which serve as an anode. The arc functions to heat the gas flow as it moves along the passageway to temperatures sufficient to ionize a portion of the gas stream and form a plasma. The heated plasma flow then moves toward the downstream end of the passageway. It is usually in this section that the material to be deposited, in powder form, is injected into the plasma flow. The material then becomes entrained in the flow and begins at least partially to melt. As the flow leaves the device through the nozzle, it is directed onto the target surface to be coated. When the plasma impacts the surface, the particles of partially or fully melted coating material bond to the surface and to each other creating the high quality bonded coating characteristic of plasma spray techniques.
Most modern plasma spray devices incorporate a convergent-divergent Laval nozzle design wherein the upstream end of the nozzle converges to a throat section from which the downstream end of the nozzle extends. The downstream end of the nozzle usually diverges from the throat. In fact, divergence of at least a portion of the downstream end is required by the laws of fluid dynamics if it is desired to achieve a supersonic plasma flow at the nozzle exit. The coating material, usually in fine powder form, typically is injected into the flow in the region of the divergent portion of the nozzle. This material enters and becomes entrained in the plasma flow and at the same time is heated by the flow so that when the flow impacts a substrate to be coated, the material bonds to its surface.
Examples of plasma spray devices such as that just described are found in the disclosures of numerous patents including U.S. Pat. Nos. 4,670,290 of Itoh et al., 5,225,6562 of Landes, 5,243,169 of Tateno et al., 5,014,915 of Simm et. al., 5,043,548 of Whitney, et al., 3,914,573 of Muehlberger, and 3,055,591 of A. P. Shepard. Most of these devices incorporate a convergent-divergent nozzle design to achieve supersonic flow, but some have cylindrical nozzles for producing subsonic flows. The typical divergent section of a plasma spray nozzle has a cone-shaped contour with straight divergent walls.
A common and serious problem inherent with plasma spray nozzles in both vacuum and air plasma spray processes is that they tend to produce overspray during the deposition process. Overspray comprises undeposited free floating powder that escapes from the plasma flow prior to deposition onto the target substrate. Overspray increases the cost of the process through wasted material and jeopardizes the integrity and quality of the coating by randomly entraining itself into the coating. The major cause of generated overspray is poor nozzle designs in commercially available plasma guns. Current supersonic nozzles have downstream ends with a conical shape and are not designed to produce ideal flow expansion at the nozzle exit. Ideal flow expansion occurs when the pressure of the exiting plasma is the same as the ambient pressure in the region of the nozzle.
The poor design of current plasma nozzles results in overspray through a variety of phenomena. For example, if the plasma flow is overexpanded at the nozzle exit; that is, if the plasma pressure is less than the ambient pressure, then a shock wave is produced at the nozzle exit, followed by an alternating series of expansion fans and shocks. Interaction between the shock waves and the flow changes the momentum, shape, and direction of the flow causing many particles (injected into the flow) to escape and become overspray. Similarly, if the plasma flow is underexpanded; i.e. the plasma pressure at the nozzle exit is greater than the ambient pressure, then expansion fans are produced at the nozzle exit, followed by an alternating series of shock waves and expansion fans. As with shock waves, interaction between expansion fans and the flow changes the momentum of and results in structure within the flow, again allowing particles to escape the flow in the form of overspray.
Conical nozzles can be designed such that the flow is ideally expanded at the nozzle exit, thus, eliminating shock and expansion phenomena. However, since the nozzle is conical, the flow at the exit plane of the nozzle embodies dynamic components that are not parallel to the axis of the nozzle. These dynamic flow components diverge and induce divergent particle trajectories as the flow traverses the space between the nozzle and the target resulting in overspray and lower particle impact velocities.
As a result of all of these phenomena, currently available plasma spray nozzles, even when designed to produce an ideally expanded flow, tend to deposit on a target substrate less than ninety percent (90%) of the coating material injected into the flow. The other ten percent (10%) or more becomes overspray. Clearly, even relatively small improvements in the efficiency of plasma spray nozzles could be critically important in reducing the expense and undesirable effects of overspray. For example, an increase in efficiency from ninety percent to ninety five percent would reduce the total volume of overspray by half. Such efficiencies, have heretofore been unattainable with conventional plasma spray nozzles.
Thus, there exists an urgent and heretofore unaddressed need for an improved plasma spray nozzle that significantly lowers the amount of overspray produced by prior art nozzles by producing a plasma flow that is both ideally expanded at the nozzle exit to eliminate shock and expansion wave phenomena and that is highly collimated to reduce the diffusing effects of divergent, dynamic components in the flow. It is to the provision of such a plasma spray nozzle that the present invention is primarily directed.